1. Field of the Invention
This invention relates to sheet molding compounds which are thickened mixtures of unsaturated polyester resin syrup, particulate fillers, reinforcing fibers, curing catalysts which remain polymerizable in sheet form. Sheet molding compounds are employed in manufacturing glass-fiber-reinforced thermoset products by compression in heated molds. Sheet molding compounds are frequently abbreviated as SMC.
2. Description of the Prior Art
Many techniques have been developed for hardening polymerizable unsaturated polyester resin syrups and reinforcing fibers without concomitantly polymerizing the unsaturated polyester resin syrup. Inorganic additives such as magnesium oxide, calcium oxide have been employed. Free maleic anhydride has been added to the compositions. Sheet molding compounds permit convenient handling of the polymerizable substance which can be introduced into heated molding elements and therein converted into thermoset fiber-reinforced products. When sheet molding compounds are employed, there is no need to introduce fibrous reinforcing materials separately into a mold cavity and also to introduce liquid polymerizable resin separately into the mold cavity. Sheet molding compounds greatly accelerate the production of glass-fiber-reinforced thermoset resinous products.
Preparation of sheet molding compounds confronts a variety of problems. One problem is that the thickening of the unpolymerized polymerizable resin may continue beyond the desired level and the resulting SMC is too hard, too brittle or otherwise unacceptable. Another problem is that inadequate thickening may occur and the resulting SMC never fully hardens to a handleable form. A further difficulty is that the hardening of the SMC takes place too slowly whereby excessive processing time is required to manufacture the SMC. A still further problem with the SMC is that the hardening process may continue at a retarded rate, requiring that such materials be used promptly and thus restricting the storage life of the SMC. The addition of diisocyanates to unsaturated polyester resins in sheet molding compounds has been described: U.S. Pat. Nos. 3,824,201; 3,933,728.